This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The purpose of this study is to test the hypothesis that in a population of adolescents and young adults with HIV infection who are being treated with tenofovir as part of an antiretroviral combination regimen, vitamin D supplementation will decrease renal phosphate loss, increase plasma phosphate, decrease plasma PTH, and improve markers of bone turnover, including a decrease in plasma N-telopeptide and BAP.